Foreign materials that remain adhered to a processed object such as an automotive body (body-in-white) subjected to a weld process include foreign materials such as iron powder and anti-corrosive oil adhered to respective press-molded parts prior to the weld process and foreign materials such as spatter adhered during the weld process. Therefore, before subjecting the automotive body to a paint process, the automotive body is placed under a shower to clean off foreign material adhered thereto and is subsequently subjected to preprocessing such as a degreasing process, a surface conditioning process, and a chemical conversion process. The pre-processed automotive body then proceeds to an electrodeposition process, a middle coat process, and a finish coat process.
A large number of processing tanks (dip tanks) are involved in preprocessing. Foreign materials introduced into a processing tank adhering to an object to be processed, in particular, foreign materials with a high specific gravity such as iron powder and spatter, settle and accumulate at the bottom part of the processing tank. In addition, an electrodeposition process also causes sedimentable pigment components of an electrodeposition paint to settle and solidify in a processing tank over a long period of use. Such iron powder, spatter, solidified pigment component and the like are churned up by a convective flow of the processing liquid and by an in-tank movement of the processed object, problematically causing iron powder, spatter, solidified pigment component and the like to re-adhere to the processed object.
A known method of removing such foreign material accumulated in a processing tank involves creating a circulatory flow in the processing tank, providing a circulatory path intake at a location on a bottom part of the processing tank where a largest amount of foreign material is retained by the circulatory flow, suctioning the retained foreign material together with processing liquid by a pump from the intake, and capturing and removing the foreign material with a filter (for example, refer to Patent Document 1).
In addition, a device is known in which a magnetic sheet is transferably arranged across an inlet, a bottom part, and an outlet of a processing tank to have iron powder settled on the bottom part of the processing tank adhere to the magnetic sheet, and the magnetic sheet is subsequently moved so that the iron powder is scraped off of the magnetic sheet by a scraper at the outside of the processing tank (for example, refer to Patent Document 2).    Patent Document 1: Japanese Patent Laid-Open No. 06-272091    Patent Document 2: Japanese Patent Laid-Open No. 2003-117458